This invention relates to flight control mechanisms for aircraft and, more particularly, relates to a bifurcated feel system for use in an aircraft having a dual path flight control system.
In order to provide stable control of an aircraft in flight it is desirable that the pilot receive a force feedback from the control surfaces which indicates the response of the aircraft to flight controls. In small aircraft force feedback can be accomplished by connecting the pilot controls directly to the aerodynamic control surfaces so that the aerodynamic forces on the control surfaces feed directly back to the pilot. However, as airplanes have gotten larger, the forces that can be applied manually by a pilot are insufficient to adequately control the aircraft and, in some cases, even to move the control surfaces. It has, therefore, become routine for the control surfaces of the aircraft to be hydraulically controlled. When the control surfaces are moved by a powered system the aerodynamic forces or "feel" previously fed back to the pilot to indicate his control of the aircraft are lost and an artificial feedback force proportional to the aerodynamic forces acting on the control surfces must be provided to enable the pilot to "feel" the action of the control surfaces. If such artificial feel were not incorporated there would be a tendency for the pilot to overcontrol, resulting in unstable flight patterns and possibly destructive conditions.
As a requirement of safety it is generally required that the flight controls be duplicated and arranged to provide dual control paths to provide for control by both the pilot and the copilot. The controls are arranged so that in the event of a failure in either of the control paths the operator of the controls in the remaining path can override the failed controls and individually fly the airplane. Since the flight control system is dual path it is necessary that feel simulation be provided to each of the control paths. In many presently used control systems a single artificial feel unit provides feel for both the pilot and copilot control paths. In certain prior art flight control systems the mechanical control paths of the pilot and copilot are directly tied together and then directly tied to the feel unit. In such a system a mechanical failure or jam in either the pilot's or copilot's control path renders the entire aircraft control system inoperative. In order to satisfy safety requirements on new aircraft, override devices are generally now provided between the pilot's and copilot's control circuits so that a failure in one or the other control circuit can be overridden by the remaining pilot and the plane can be flown on the remaining operative control system.
Even with the override systems installed, certain disadvantages remain in a flight control system having a single conventional feel unit. It is necessary that the feel unit be directly connected to only one of the control paths, for example, the pilot's. Therefore, the copilot receives feel simulation by way of the override devices coupling the copilot's control path to the pilot's control path. Since the copilot applies force to the feel system only through the overrides, the breakout force of the overrides must be very high in order to prevent inadvertent breakout during normal operations from the copilot's side. Therefore, very high copilot breakout control forces result from a jam of the pilot's control circuit. This is an undesirable condition since breakout forces should be as low as possible to provide an easily controlled aircraft. Also, in the system described above, if the copilot's control path jams the pilot must apply the high breakout force added to the full feel force for control of the control surface. The pilot's and copilot's jammed override control forces therefore are dissimilar. The disparity in override control forces is undesirable from a training standpoint since it requires variations in training between the copilot and pilot and retraining when those roles are reversed.
The problems mentioned above have been somewhat alleviated by arrangements in which separate feel units are provided for the pilot and the copilot. Such a system is symmetrical and has reasonable jam breakout forces but such a system has the undesirable feature of increasing weight of the aircraft and also the cost, since two complete feel generating systems must be installed. Also, having two feel units in the system adds friction to the control system. The control surfaces of the aircraft are very susceptible to friction, particularly the elevator system. Also, there is a problem of synchronizing the rigging of the duplicate control systems.
It is therefore an object of the present invention to provide a single feel unit which will simultaneously provide control surface feedback or "feel" to both the pilot and copilot control paths of a dual path aircraft control system.
It is a further object of the present invention to provide such a feel unit which introduces substantially the same amount of friction into the system as would be introduced by a single feel unit attached to only the pilot control system.
It is an object of the present invention to provide a feel unit which simultaneously provides feel to both the pilot and copilot control systems and can be operated with relatively equal and reasonable jam breakout forces for each control path.